1. Field of the Invention
This invention relates generally to an endless power transmission belt with wear-protecting reinforcing fabric or jacket on at least one surface, more particularly to an endless toothed belt with a fabric on the tooth side which has a seam extending over more than two teeth, and to a method of making the same involving helically-wrapping a continuous fabric strip.
2. Description of the Prior Art
Representative of the art of toothed power transmission belts and methods of making the same is U.S. Pat. No. 4,235,119 to Wetzel. Wetzel discloses three known methods of manufacturing toothed belts including the extruded tooth method, the tooth preform method and the flow through method. All methods disclosed therein use a sheet of fabric wrapped on a mandrel (i.e., a grooved cylindrical mold) with the ends of the fabric sewn together or otherwise united in a butt joint or lap joint, whether the fabric is square woven, bias-cut, stress-relieved, or stretchable. The sewn fabric sleeve or tube is often called a “jacket” and the joint may be called a “splice.” The butt joint is preferably (according to Wetzel) located over the ridges between the grooves in the outer circumference of the mold. As illustrated in FIG. 1, the jacket becomes the tooth cover 12 for the teeth 18 in the finished belt 10, with the butt joint 15 located preferably (according to Wetzel) in a land region 19 of the belt, i.e. the thin section between two teeth. Belt 10 also generally includes one or more tensile cord 16 spirally wound and elastomeric belt body 14 which fills out the teeth and surrounds the tensile cord. The flow through method includes placing the fabric tube on the mandrel, spirally wrapping the cord onto the fabric, wrapping a layer of rubber onto the cord layer, then curing under pressure so that the rubber flows between adjacent cords to fill out the teeth, pressing the fabric into the mandrel grooves. Joining or uniting the fabric ends is necessary so that the jacket stays in place on the mandrel during the cord winding step, which generates centrifugal forces that can cause the jacket to expand.
Also representative of the art of toothed power transmission belts and methods of making the same is U.S. Pat. No. 3,756,091 to Miller. Miller discloses that there is a tendency for those splices of the jacket which lie entirely in the root and land area of a belt tooth to break in use caused by the reduced strength of the jacket. To eliminate this problem, Miller teaches, as illustrated by belt 20 in FIG. 2, that the jacket 12 should be bias cut so that the minimum angle that splice 25 makes with the belt side is such that if the splice starts at the point where the root 22 of a tooth 18 blends with its land area 19 it will end, when viewed in cross section, at the equivalent blend point on the other side of the same tooth, and preferably, as shown, the splice 25 is at such an angle that the splice spans two teeth 18.
Herein, the sewn fabric splices illustrated in FIGS. 1 and 2 will be referred to as “transverse” and “bias” splices or joints, respectively. A transverse splice runs from one side of the belt to the other side of the belt generally within the space of a single tooth pitch, although it may wander more than that if not carefully aligned during building. The bias splice of FIG. 2 typically might make an angle with the belt edge of about 45° and span one or two tooth pitches. Conventional splice joining methods for a sheet of fabric include sewing, fusing, laminating, bonding, and the like, and the joining step can be difficult to carry out without folds, gaps, or other potential defects. These splices are generally considered weaker than the rest of the fabric, and the tooth or land in which the joint resides may be the weakest portion of the belt to certain stresses. The bias splice and transverse splice can both also result in a lot of wasted material. Splicing also is a labor intensive operation. It is currently generally preferred in the art to have a transverse jacket splice which is located in and follows along the tip of a single tooth (contrary to Wetzel's teaching mentioned above). Such splice location is difficult in practice.
U.S. Pat. No. 4,395,298 to Wetzel, et al., U.S. Pat. No. 4,444,621 to Marsh, et al., and U.S. Pat. No. 5,421,927 to Macchiarulo, et al., disclose methods for making long length toothed belts based on first forming a tubular sleeve comprising cover fabric, helically wrapped tensile cord, and elastomeric belt body material, then spirally cutting the sleeve to make a continuous strip of toothed-belt material, helically wrapping the strip around two wrapping drums which align the teeth and control the belt length, and fusing the adjacent loops of helically wrapped strip to form a new, longer belt or tubular sleeve. As a result of this helical cutting and wrapping process, the toothed belt exhibits a cover fabric structured in the form of a narrow continuous strip forming loops disposed in side-by-side relation and mating at the respective opposite edges. Marsh, et al., teaches that the helix angle of the spiral cut should be in the approximate range of 0.1 to 0.5 degrees, to provide a strip width in the approximate range 5 to 20 mm. The cutting angle is approximately equal to the helical angle of the tensile cords. This small angle is necessary to prevent or minimize cutting of the cords, which would reduce the tensile strength of the resulting belt.